14 OUTLINES OF PHYSICAL CHEMISTRY
��Determination op Atomic Weights
The atom of an element is the smallest quantity of it which can enter«into chemical combination. The atomic weight of an element is deduced from a study of all the volatile compounds into the constitution of which the element dan enter.
If, for example, we are concerned with the atomic weight of chlorine, we know the following volatile com- pounds containing this element :
Silicon Chloride. — Molecular weight experimentally found, 169*5 ; that is, the molecule weighs so many times more than the atom of hydrogen. Analysis shows th&t in 169*5 parts of silicon chloride there are 28 parts of silicon, and 141*5 parts of chlorine. In the same way we find the following data for some other volatile chlorides :
1141*50 chlorine
Phosphorous chloride. . 1880oi ^?S P ^ h0rUS
1106*10 chlorine
��Mercuric chloride
��• - 27000 1 1 ^"7f mercury
( 70-74 chlorine
I 85-87 chlorine
•
Besides these we know a large number of other com- pounds containing chlorine which are volatile. But not one of them contains in its molecule a quantity of chlorine less than 85*37 times the weight of one atom of hydrogen. This number, therefore, represents the atomic weight of chlorine, i.e. the relative weight of the smallest possible particle of this element which can exist in any compound.
The atomic weight thus determined may be too large, but it cannot be too small. The probability of its correct- ness increases as the number of compounds examined from which it is derived increases. The elements of low atomic
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